President Obama issued a letter to the Senate on Sunday pledging to fully develop a U.S. missile defense system in Europe, as part of a final offensive to relieve concerns about the nuclear arms pact with Russia as it moves toward a final vote.

The letter reiterated administration policy but was an especially extensive and detailed statement on missile defense by the president. Parts of it were read aloud by Sen. John F. Kerry (D-Mass.) shortly before a vote on an amendment that could have killed the treaty. That amendment was defeated, 59 to 37.

Sen. Bob Corker (R-Tenn.), who has been leaning toward supporting the New Strategic Arms Reduction Treaty (START), took to the floor to welcome the president's letter. "A number of people on our side of the aisle have asked for it," he said.

Missile defense has emerged as the greatest point of contention over the treaty. Although the pact is focused on arms reductions and verification, its preamble briefly mentions an "interrelationship" between nuclear weapons and missile defense.

Russia has said it could withdraw from the pact if the U.S. missile defense system becomes ambitious enough to fend off its arsenal. U.S. officials say their intentions are more modest - a system aimed at countries such as Iran and North Korea.

Some Republicans worry that the missile defense language, although not legally binding, could give the Russians a pretext to pressure the U.S. government.

"In today's world, there are so many new and constantly evolving threats. The United States can't be limited" in deploying missile defenses, said Sen. John McCain (R-Ariz.). He was one of the sponsors of the amendment to remove the language.

Trimming that, however, would send the treaty back to the negotiating table.

Obama said in the letter that New START "places no limitations on the development or deployment of our missile defense programs." The president said that he "will take every action available to me to support the deployment of all four phases" of a missile defense system in Europe.

Senators had questioned whether the White House might hold off on developing the last phase of the program, which would be aimed at stopping U.S.-bound Iranian missiles, for fear of antagonizing Russia.

Sen. Jon Kyl (R-Ariz.), who has led opposition to the treaty, dismissed the letter as a "last-ditch effort there to win votes, or preclude an amendment from passing."

Obama devoted his weekly radio address to the treaty, saying that it "isn't about winning a victory for an administration . . . it's about the safety and security of the United States of America."